A cascaded photovoltaic grid-connected inverter includes n cascaded inverter units (such as an H-bridge inverter unit), and a topological structure of the cascaded photovoltaic grid-connected inverter is shown in FIG. 1. A direct current side of each of the inverter units is connected with an independent photovoltaic array, and alternating current sides of the inverter units are connected in series, and then connected with a power grid through a filtering inductor L.
A conventional control strategy of the cascaded photovoltaic grid-connected inverter is described as follows. Each of the inverter units independently performs maximum power point tracking, stabilizes a direct current bus voltage and uploads a power signal to a total controller. The total controller calculates a grid-connected current instruction value based on all power signals, and then achieves an effective control of the current of the power grid by using a PR controller.
However, the conventional control strategy is easy to result in an inverter unit over-modulation phenomenon in a case that output active powers between the inverter units are seriously unbalanced. For example, when an output power of a photovoltaic array corresponding to an inverter unit #1 decreases due to uneven lighting or other factors, an output active power Pi of the inverter unit #1 is inevitably reduced according to a principle of active power conservation, thereby causing decrease in a grid-connected current effective value. After the grid-connected current effective value is decreased, output voltages of inverter units #2 to # n are respectively increased in order to maintain the active power conservation of the inverter units #2 to # n. In a case that an output voltage of a certain inverter unit exceeds a direct current bus voltage of the inverter unit (that is, a modulation degree of the inverter unit is greater than 1), an over-modulation phenomenon occurs in the invert unit, thereby resulting in a negative impact on the grid-connected operation stability of the cascaded photovoltaic grid-connected inverter.